In-mold coating is a known technique for decorating or priming the surface of an article formed in a matched die mold under heat and pressure. A coating powder, which can be an unsaturated polyester formulation, is applied electrostatically to the interior wall of the mold. The powder melts and coalesces on the hot mold surface to produce a uniform coating. The fill resin, which constitutes the bulk of the finished article, is then inserted in the mold and heat and pressure is applied. When the cures of the coating and fill resin have advanced sufficiently and said resins are integral, the mold is opened and the molding is removed. It has been found desirable that the coating and fill resins be cross-linked across the interface between them to maximize the adhesion of one to the other.
The following patents relate to in-mold coating techniques of the type used in this invention:
U.S. Pat. Nos. 3,216,877, 4,205,028, 4,228,113, 4,287,310, 4,315,884, 4,499,235, 4,873,274.
British Patent Specification No. 1,420,867 teaches an analagous process except that the fill resin is first formed in the mold and apparently cured, then removed from the mold. The mold is powder coated and the formed resin is returned to the mold and remolded to apply the coating. The teachings of U.S. Pat. No. 4,873,274 about combinations of initiators is hereby incorporated herein by reference.
The surface hardness, i.e., the scratch resistance, and smoothness of molded articles of reinforced polyester on the market still leave something to be desired. For example, the manufacturers of molded sinks and bathtubs usually warn the users that the use of abrasive cleaners will mar the pleasing appearance of their surfaces. The scratching of such surfaces by the movement of accessories thereon has also been a problem. Elimination of the phenomenon known as telegraphing, i.e., the visibility of reinforcing glass fibers under the surface, also has continued to be a problem up to the time of this invention.
The surface hardness can be increased by the use of more highly functional monomers such as triallylcyanurate, diallylphthalate, divinylphthalate, and other multi-functional monomers or polymers in the coating powder composition. The use of higher levels of these materials, however, increases the reactivity of the coating powder, rendering it less stable on room temperature storage and causing premature polymerization when applied to the mold. Tearing of the powder coating results when such a powder is placed on the mold surface, the fill resin is charged to the mold, and the mold is closed under pressure. Also, most allylic monomers and prepolymers have a low vapor pressure and an undesirable odor which produce undesirable effects during application and molding. The presence of such allylic monomers in the coating powder also lowers the T.sub.g of the coating powder, causing it to block during storage or shipment. Further, these expensive monomers increase the cost of the in-mold coating powder.